The reinforcement of tires, and especially of heavy-goods vehicle tires, is at the present time—and most often—formed from a stack of one or more plies conventionally denoted as “carcass plies”, “crown plies”, etc. This way of denoting the reinforcements derives from the manufacturing process, which consists in producing a series of semi-finished products in the form of plies, which are provided with often longitudinal thread-like reinforcing members that are subsequently assembled or stacked so as to build a tire blank. The plies are produced flat, with large dimensions, and are then cut up according to the dimensions of a given product. The assembly of the plies is also carried out, firstly, approximately flat. The blank thus produced then undergoes a forming operation so as to adopt the typical toroidal profile of tires. The semi-finished or “finish” products are then applied to the blank so as to obtain a product ready to be vulcanized.
Such a “conventional” process involves, in particular in respect of the phase of manufacturing the tire blank, the use of an anchoring element (generally a bead wire) used to anchor or retain the carcass reinforcement in the bead zone of the tire. Thus, for this type of process, a portion of all of the plies making up the carcass reinforcement (or only one part thereof) is turned up around a bead wire placed in the bead of the tire. This anchors the carcass reinforcement in the bead.
The generalization in industry of this type of conventional process, despite many variations in the way in which the plies and the assemblies are produced, has led those skilled in the art to use a vocabulary taken from the process: hence the generally accepted terminology comprising, in particular, the terms “plies”, “carcass”, “bead wire”, “shaping”, to denote the transition from a flat profile to a toroidal profile, etc.
Nowadays, there are tires which strictly speaking do not have “plies” or “bead wires” according to the above definitions. For example, document EP 0 582 196 discloses tires manufactured without the aid of semi-finished products in the form of plies. For example, the reinforcing elements of the various reinforcement structures are applied directly to the adjacent layers of rubber compounds, the whole assembly being applied in successive layers on a toroidal core, the shape of which results directly in a profile similar to the final profile of the tire under manufacture. Thus, in this case, there are no longer “semi-finished” products or “plies” or “bead wires”. The base products, such as the rubber compounds and the reinforcing elements in the form of threads or filaments, are directly applied to the core. Since this core is toroidal in shape, it is no longer necessary to form the blank in order to go from a flat profile to a torus-shaped profile.
Moreover, the tires disclosed in the above document do not have the “conventional” carcass ply upturn around a bead wire. This type of anchoring is replaced with an arrangement in which circumferential threads are placed adjacent to said sidewall reinforcement structure, the whole assembly being embedded in an anchoring or bonding rubber compound.
There are also assembly processes on a toroidal core using semi-finished products especially suitable for rapid, effective and simple laying on a central core. Finally, it is also possible to use a hybrid comprising both certain semi-finished products, in order to produce certain architectural aspects (such as plies, bead wires, etc.), whereas others are produced by direct application of compounds and/or reinforcing elements.
In the present document, to take into account recent technological developments both in the manufacturing field and in product design, the conventional terms such as “plies”, “bead wires”, etc. are advantageously replaced with neutral terms or terms that are independent of the type of process used. Thus, the term “carcass-type reinforcing member” or “sidewall reinforcing member” is valid for denoting the reinforcing elements of a carcass ply in the conventional process, and the corresponding reinforcing elements, which are in general applied to the sidewalls, of a tire built using a process without semi-finished products. As regards the term “anchoring zone”, this may denote just as well the “conventional” carcass ply upturn around a bead wire of a conventional process as the assembly formed by the circumferential reinforcing elements, the rubber compound and the adjacent sidewall reinforcing portions of a bottom zone produced by a process with application on a toroidal core.
In general in heavy-goods vehicle tires, the carcass reinforcement is anchored on either side in the region of the bead and is surmounted radially by a crown reinforcement consisting of at least two superposed layers and formed from threads or cords that are parallel in each layer and crossed from one layer to the next, making angles of between 10° and 45° with the circumferential direction. Said working layers, forming the working reinforcement, may be covered with at least one protective layer formed from advantageously metal extensible reinforcing elements, called elastic elements. The crown reinforcement may also comprise a layer of low-extensibility metal threads or cords making an angle of between 45° and 90° with the circumferential direction, this ply, called triangulation ply, being located radially between the carcass reinforcement and the first crown ply called the working ply, these being formed from parallel threads or cords at angles of at most equal to 45° in absolute value. The triangulation ply forms, with at least said working ply, a triangulated reinforcement which undergoes, when subjected to the various stresses, little deformation, the essential role of the triangulation ply being to take up the transverse compressive forces to which all of the reinforcing elements in the crown region of the tire are subjected.
In the case of heavy-goods vehicle tires, a single protective layer is usually present and its protecting elements are, in most cases, oriented in the same direction and at the same angle in absolute value as those of the reinforcing elements of the radially outermost, and therefore radially adjacent, working layer. In the case of civil engineering vehicle tires, intended for running on more or less uneven ground, the presence of two protective layers is advantageous, the reinforcing elements being crossed from one layer to the next and the reinforcing elements of the radially inner protective layer being crossed with the inextensible reinforcing elements of the radially outer working layer adjacent to said radially inner protective layer.
The circumferential direction, or longitudinal direction, of the tire is the direction corresponding to the periphery of the tire and defined by the running direction of the tire.
The transverse or axial direction of the tire is parallel to the rotation axis of the tire.
The radial direction is a direction cutting the rotation axis of the tire and perpendicular thereto.
The rotation axis of the tire is the axis about which it rotates in normal use.
A radial or meridian plane is a plane that contains the rotation axis of the tire.
The circumferential median, or equatorial, plane is a plane perpendicular to the rotation axis of the tire and that divides the tire into two halves.
Certain current “road” tires are intended to run at high speed on increasingly long journeys, because of the improvements in road networks and the growth of motorway networks throughout the world. All the conditions, under which such a tire is called upon to run, without doubt enable the tire to be run for a larger number of kilometers, since the wear of the tire is less. However, the endurance of this tire is prejudiced. To permit one or even two retreading operations on such tires, so as to extend their lifetime, it is necessary to preserve a structure and especially a carcass reinforcement with endurance properties which are sufficient to withstand said retreading operations.
Prolonged running under particularly severe conditions of tires thus constructed effectively introduces limits in terms of endurance of these tires.
The elements of the carcass reinforcement are in particular subjected to flexural and compressive stresses during running which adversely affect their endurance. The cords that make up the reinforcing elements of the carcass layers are in fact subjected to large stresses when the tires are running, especially to repeated flexural stresses or variations in curvature, leading to friction between the threads, and therefore wear and fatigue: this phenomenon is termed “fatigue fretting”.
To fulfill their function of strengthening the carcass reinforcement of the tire, said cords must firstly have good flexibility and a high endurance in flexure, which means in particular that their threads have to have a relatively small diameter, preferably less than 0.28 mm, more preferably less than 0.25 mm, generally smaller than that of the threads used in conventional cords for the crown reinforcements of tires.
The cords of the carcass reinforcement are also subjected to the phenomenon of “fatigue-corrosion” due to the very nature of the cords, which promote the passage of corrosive agents such as oxygen and moisture or even drain said agents. Specifically, air or water penetrating the tire, for example as a result of degradation following a cut or more simply because of the permeability, albeit low, of the inner surface of the tire, may be conveyed by the channels formed within the cords because of their very structure.
All these fatigue phenomena, which are generally grouped together under the generic term “fatigue-fretting-corrosion”, are the cause of progressive degradation of the mechanical properties of the cords and may, under the severest running conditions, affect the lifetime of said cords.
To improve the endurance of these cords of the carcass reinforcement, it is known in particular to increase the thickness of the rubber layer that forms the internal wall of the cavity of the tire in order to minimize the permeability of said layer. This layer is usually composed partly of a butyl rubber so as to better seal the tire. This type of material has the drawback of increasing the cost of the tire.
It is also known to modify the construction of said cords so as in particular to increase their penetrability by the rubber and thus limit or even eliminate the passage of oxidizing agents via the channels formed within the cords. Tires thus produced demonstrate problems of air pockets appearing during tire manufacture.
Specifically, the various manufacturing steps result in the formation of occluded air pockets. In the case of tires having a carcass reinforcement formed from cords, the structure of which forms channels able to conduct air, these air pockets disappear because the air diffuses into the materials, especially through said channels existing within the cords. In the case of tires having a carcass reinforcement formed from cords having a structure which is highly penetrated by the rubber, these air pockets remain after the manufacturing steps. What occurs is merely a displacement of these air pockets during the step of curing the tire, these pockets moving towards regions where a low pressure is exerted. The displacement of the air takes place along the carcass reinforcement, following passages that exist between the reinforcing elements, the layers of rubber compound covering the reinforcing elements that form reinforcing regions parallel to the reinforcing elements before the step of curing the tire. These reinforcing zones thus allow the air to be moved slightly depending on the pressure that is exerted on the regions where the air pockets are. The pressure or the pressure variations arise particularly during the step of curing the tire or else during the shaping step, if this exists.
The appearance of these air pockets is usually unacceptable, depending on their location, and may require the tires to be scrapped, since said air pockets may become regions of weakness in the tire. The manufacturing costs therefore become unacceptable simply because of the poor production yields.